1. Field of the Invention
This invention relates to an optical fiber whose mode field diameter can be reduced at a desired portion, a method for fabricating the same, a mode field diameter conversion optical fiber using the optical fiber, and a method for converting a mode field diameter of the mode field diameter conversion optical fiber.
1. Related Background Art
A problem involved in inserting optical parts between optical fiber ends which oppose each other is that a gap is formed between the optical fibers. The size of the gap corresponds to a thickness of the optical parts. The presence of this gap acts to decrease light transmittance. The amount of decrease in light transmittance depends on the size of the gap, that is, a distance between the optical fiber ends opposing each other, and a mode field diameter of the optical fiber 1. This is shown in FIGS. 1A and 1B. The relationship between the light transmittance, the distance between optical fiber ends opposing each other, and the mode field diameter is expressed by Formula 1. EQU T=1/[1+a(.lambda.d/w.sup.2).sup.2 ] (1)
where
T: a light transmittance
a: a constant determined by an optical fiber
.lambda.: a wavelength of transmission light
d: a distance between optical fiber ends opposing each other
w: a mode field diameter of the optical fiber
As apparent from Formula 1, in a transmission unit using light of a specific wavelength, to decrease the light transmission loss caused by the distance between opposing optical fiber ends, it is preferable that the mode field diameter of both optical fibers at their opposing ends be large. In long-distance transmission using single-mode optical fibers, however, it is preferable that their mode field diameter be small in view of light losses due to their bends, twists, etc.
The following two methods are proposed for inserting optical parts between optical fibers and suppressing the decrease of light transmittance.
a) A small-mode field diameter optical fiber is formed, and the mode field diameter is expanded at a portion of the optical fiber. The optical fiber is divided at the portion of the expanded mode field diameter, and optical parts are inserted between the divided optical fibers. Optical fibers for long-distance transmission uses are connected to the divided optical fibers at the end faces other than the divided end faces.
b) An optical fiber of a large-mode field diameter is formed, and the mode field diameter is reduced at both ends of the optical fiber. The optical fiber is divided at the portion of the large mode field diameter, and optical parts are inserted between the divided optical fibers opposing each other. Optical fibers for long-distance transmission uses are connected to the divided optical fibers at the end faces of the reduced mode field diameter.
The method (a) has been conventionally used because of its practicality. Specifically, to convert a mode field diameter, the following two methods have been practiced.
1) A portion of an optical fiber is heated by a heater 4 or other heating means and elongated by applying tension along arrow A to reduce a core diameter, whereby its mode field diameter is increased (FIG. 2).
2) A portion of an optical fiber is heated to thermally diffuse a dopant added to-the core for improving its refractive index so as to effectively lower a refractive index difference between the core 2 and the cladding 3, whereby a mode field diameter is expanded (FIG. 3).
In the method (1), since optical fibers having an outer diameter as small as 125 .mu.m (CCITT Standards) are elongated, mode field diameters can be expanded only over regions of short lengths. The regions of short lengths can accommodate only a small number of optical parts, but cannot accommodate a large number of optical parts. The method (2) can solve the problem of obtaining lengthy regions of expanded mode field diameters by increasing heating regions. But the method (2) requires a longer heating time (some hours), which results in problems of low productivity and deformation of optical fibers.